Line filters are used to prevent excessive noise from being conducted between electronic equipment and AC line (Document 1). FIG. 1 shows the use of a common-mode filter (1a) between the AC line (1) and a power converter (2) (Document 1). In FIG. 1, the direction of common-mode noise is from the load and into the filter, and the noise common to both lines becomes attenuated, making the resulting common-mode output of the filter onto the AC line negligible (Document 1).
FIG. 2 shows a common-mode inductor. The design of a common-mode filter is the design of two identical differential filters, one for each of the two polarity lines with the inductors of each side coupled by a single core (Document 1). For a differential input current ((A; 3) to (B; 4) through L1 (5) and (B) to (A) through L2 (6)), the net magnetic flux, coupled between the two inductors L1 and L2, is zero (Document 1). Any inductance encountered by the differential input signal is the result of imperfect coupling of the two chokes, and they perform as independent components with their leakage inductances responding to the differential signal (Document 1). The leakage inductances attenuate the differential signal (Document 1). When the inductors encounter an identical signal of the same polarity referred to ground (common-mode signal), they each contribute a net, non-zero flux in the shared core, the inductors thus perform as independent components with their mutual inductance responding to the common signal, and the mutual inductance then attenuates this common signal (Document 1).
The simplest and least expensive filter is a first order filter (Document 1). This first order filter uses a single reactive component to store certain bands of a spectral energy without passing this energy to the load (Document 1). In case of a low pass common-mode filter, a common-mode choke is the reactive element employed (Document 1). Here, the value of inductance (L; 7) required of the choke is the load (in Ohms; 8) divided by the radian frequency at and above which the signal is to be attenuated (Document 1), which is represented by the following relationship:
      L    =                  R        L            ω        ,wherein ω is a radian frequency, and RL is the noise load resistance. FIG. 3 shows a first order common-mode filter.
A second order filter is designed to use two reactive components and has two advantages over the first order filter: provides 12 dB over octave attenuation after the cutoff point; and provides greater attenuation at frequencies above inductor self-resonance (Document 1). FIG. 4 shows a second order common-mode filter. The second order common-mode filter in FIG. 4 has the following relationship (relationship between VCMOUT (9) and VCMIN (10)):
                              V                      CM            OUT                          ⁡                  (          s          )                                      V                      CM            IN                          ⁡                  (          s          )                      =                  1                  1          +                                    L                              R                L                                      ⁢            s                    +                      LCs            2                              =                        1                      1            +                          j              ⁢                                                          ⁢              2              ⁢              ζ              ⁢                              ω                                  ω                  n                                                      -                                          (                                  ω                                      ω                    n                                                  )                            2                                      =                  1                      1            -                          LC              ⁢                                                          ⁢                              ω                2                                      +                          j              ⁢                                                          ⁢                              ω                ⁡                                  (                                      L                                          R                      L                                                        )                                                                          ,
where ω is a radian frequency, RL is the noise load resistance,
            ω      n        =                  1                  LC                    ⁢      m        and      ζ    =                  L                  2          ⁢                      R            L                    ⁢                      LC                              .      
A third order filter is designed to use three reactive components and yields an attenuation of 18 db per octave above the cutoff point. However, since it has to use three reactive components, it is highly expensive (Document 1). FIG. 5 shows a third order common-mode filter. The third order common-mode filter in FIG. 5 has the following relationship (relationship between VCMOUT (11) and VCMIN (12)):
                    V                  CM          OUT                    ⁡              (        s        )                            V                  CM          IN                    ⁡              (        s        )              =                    (                              R            L                                              R              L                        +                                          L                2                            ⁢              s                                      )            ⁢              (                                                            R                L                            +                                                L                                      2                    ⁢                                                                                                                ⁢                s                                      sC                                                                                R                  L                                +                                                      L                    2                                    ⁢                  s                                            sC                        +                                          R                L                            ⁢                              L                1                            ⁢              s                        +                                          L                2                            ⁢                              L                1                            ⁢              s                        +                                          L                2                            ⁢                              L                1                            ⁢                              s                2                                      +                                                            L                  1                                ⁢                s                            sC                                      )              =                  1                  1          +                                                                      L                  1                                +                                  L                  2                                                            R                L                                      ⁢            s                    +                                    L              1                        ⁢                          Cs              2                                +                                                                      L                  1                                ⁢                                  L                  2                                ⁢                C                                            R                L                                      ⁢                          s              3                                          .      
It is noted that filters with higher than third order are general cost-prohibitive (Document 1).
In order to consolidate and be complied with the EMC (Electromagnetic Compatibility) standard, a filter targeted to reduce the quantity of unique fixed-corner-frequency physical LC (inductor-capacitor) filter is necessary. The filter needs to have an adjustable corner-frequency frequency. Also, a corner-frequency of the filter needs to be adjusted to emulate multiple fixed-corner-frequency filters.
However, none of the filters in the market is related to current- or voltage-tunable EMC filters. While there is a filter in the market, which uses Varactor diodes to provide an adjustable capacitance to help tune radio-frequency receivers, the proposed filter changes the value of inductance, not capacitance to tune the filter, and the proposed filter is applied to implement the EMC filtering, not radio-wave reception.
In order to solve the above-mentioned and/or other problems, the present invention describes a tunable LC filter, which reduces the number of unique fixed-corner-frequency physical LC filters by providing an adjustable corner-frequency characteristic, and which can be tuned by using fixed or variable current (DC/AC signals) to change an inductance of a common-mode choke.